Method of working aluminum and product thereof



July 15, 1 941.

7 T. L. FRITZLENN METHOD OF WORKING ALUMINUM AND Pnobuc'r THEREOF FiledAug. 23, 1959 n 6 W 9. MM a m Z n -sound surface on the product.

Patented July 15, 1941 METHOD OF WORKING ALUMINUM AND PRODUCT THEREOFThomas L. Fritzlen, West Lafayette, Incl, assignor to Aluminum Companyof America, Pittsburgh, Pa., a corporation of Pennsylvania.

Application August 23, 1939, Serial No. 291,538

11 Claims.

This invention relates to a method for improving the workability of thatclass of aluminum base alloys which respond to solution heat treatmentfor increasing their mechanical .properties, and it relates morespecifically to such method in the art of extrusion. The invention isparticularly concerned with methods of extruding aluminum base alloys ofthis class, especially those containing copper as the main solidsolutionforming element. The term aluminum base alloys as used in thisspecification and in the appended claims means alloys containing morethan about 80 per cent aluminum.

According to the usual practice of extruding these alloys, cast ingotsor billets of the alloys are heated to the proper extrusion temperatureand extruded into the desired shapes. The extrusion is accomplished bymeans of an extrusion press which generally comprises a heated cylinderfor receiving the heated ingot, a die at one end of said cylinder whichhas an opening of the desired shape, and a ram at the other end whichforces These factors also to some extent indirectly affect the qualityof such products. I will briefly mention several of the more importantof these factors, since an understanding of their significance isnecessary for a proper appreciation of the advantages of my invention.

It is evident that economic considerations favor an increase inextrusion speed, which speed is usually referred to in terms of theprogression of extruded materialfrom the die in linear feet per'minute.As this speed is increased, however, there arises an increasing tendencytowards what 1 is known as pickup, which may be defined as a tendency ofthe extruded metal to adhere or bond to the bearing surface of the die.Thisnot only decreases'extrusion speed but also causes grooves andsurface irregularities on the extruded shapes; Generally speaking, theextrusion press is of ample capacity to force the'metal through the dieat greater speeds than actually employed, but the operating speed isnecessarily llmited to that which will produce an acceptable Theattainment of a desired extrusion-speed may be prevented by anothersurface condition, usually termed surface cracking. Under someconditions minute cracks may appear, which grow larger as extrusionspeed is increased until the product eitherruptures completely, or isotherwise unsatisfactory for commercial purposes.

For these and other reasons it has been found I that there existdefinite maximum speeds at which alloys of the type herein underconsideration can be satisfactorily extruded, and one of the problems ofthis art has been to develop means for permitting an increase in theextrusion speeds and for improving the efliciency of the process whilemaintaining a high standard of quality for the product.

It has been found that the structure of ingots in the as-cast conditioncommonly used for making extruded products is characterized by highlycored dendritic grains with a considerable amount of eutectic network inthe interstices of the dendrites, said eutectic network being composedof both soluble and insoluble constituents,

and that this structure offers resistance to work- 'ing and to plasticflow. To acertain extent, of

course, this varies with the composition of the alloy. v,

It is a principal object of this invention to provide a process fortreating the aforementioned ingots of aluminum base'alloys having a.highly Another object is to provide such a process of extrusion by whichpickup is minimized, and in which the conventional extrusion presses maybe used. Another-object is to provide a process whereby alloys may beextruded at increased speeds yet not inconsistent with satisfactorycommercial quality of the product. Another ob- 'ject is to provide aprocess whereby surface cracking is substantially eliminated; Otherobjects and advantages will be apparent from thefollowing explanation ofthe invention.

The invention resides in the discovery that by subjecting the aluminumbase alloy ingots to.

an extended heating and a subsequent slow cooling to a temperature atwhich practically no further structural change occurs and-then re-'heating to the extrusion temperature, a change in the structure of thealloy occurs which is accompanied by a decrease in resistance to plasticdeformation which permits extrusion at higher speeds and/or lowertemperatures. The

altered structure, resulting from the thermal treatment, ischaracterized by a generaldistribution throughout the alloy of a largenumber of relatively, large precipitated particles of the solubleconstituents both within the grains and at the grain boundaries. Byrelatively large precipitated particles is meant precipitated particleslarger than those formed when the alloy is subjected to the well knownsolution heat treatment and precipitation hardening methods, wherein itis heated, quenched and aged to impart hardness and strength thereto. Ihave found that ingots possessing such a microstructure can be extrudedat greater speeds and at lower temperatures than ingots which have beenonly heated up to the extrusion temperature in the usual manner. I havealso found that at any stated temperature the billet can be extruded athigher speed,while still of good quality and free from cracks.

Describing in more detail the process of the present invention, the castingots are homogenized by heating them at a temperature and for a periodof time required to dissolve substantially all of the solubleconstituents. The term homogenize as herein employed refers to thecondition where a substantial portion of the soluble constituents of thealloy have been dissolved and is usually accompanied by a virtualdisappearance of the cored dendritic structure. This operation causesundissolved particles of the soluble elements located at the grainboundaries and in the eutectic network-to go into solid solution. Theeffect of this heating step upon the structure is readily apparent froma comparison of the accompanying drawing, in which:

Fig. 1 is a photomicrograph at 100:: of an aluminum base alloycontaining 4.0 per cent copper, 0.5 per cent manganese, and 0.5 per centmagnesium, which has been cast and cooled by conventional methods. Thehighly cored nature of the dendritic structure is exhibited by thedarker portions of the grains near their edges, and a large amount ofeutectic substance at the grain boundaries is clearly visible;

Fig. 2 is a photomicrograph at 100K of the same alloy after it had beenheated according to this invention, at a temperature of from 905 to 935F. for eight hours and subjected to a drastic quench to retain thestructure existing at said temperature, and shows the homogenization ofthe structure, as exhibited by a reduction of the shaded areas of thegrains, and by the reduction of the amount of eutectic substance at thegrain boundaries.

In general, aluminum base alloys containing about 2.5 to about 6.0 percent copper, from about 0.1 to about 1.0 per cent manganese, and fromabout 0.1 to about 2.0 .per cent magnesium, require heating for a periodof several hours depending on the size of the ingots, usually abouteight hours, at a temperature of from about 890 F. to about 950 F. afterthe metal has reached a uniform temperature within this range to producethe desired degree of solution as shown in Fig. 2, although somesolution is obtained in less time. The alloys may be heated at thesetemperatures for longer periods than eight hours, depending on therelative amount of copper, magnesium and silicon present, the size ofthe ingot being heated, and the size of the load where a batch of ingotsare treated at one time. Forother alloys containing magnesium, zinc, orsilicon as the chief added alloying element, for example, the period ofheating to dissolve the soluble constituents may differ from thatrequired for the foregoing alloys containing copper as the chief addedalloying element. Furthermore, the composition of the alloysolution inthe form of the aforementionedlarge particles uniformly distributedthroughout the alloy. It is preferred to continue the cooling down to atemperature at which substantially no further precipitation occurs inorder to secure maximum precipitation. In most cases this condition hasbeen reached when the alloy has been cooled to close to ordinarytemperatures. In the case of copper-containing alloys, such as thosereferred to above, this precipitate is mainly 011A]:-

The photomi-crograph of Fig. 3 is of the same alloy at 1002! as that ofFigs. 1 and 2 after it has been heated to the condition shown in Fig. 2and slowly cooled to room temperature. Fig. 3 shows clearly thecondition of the alloy after this precipitation step. The uniformlyprecipitated particles and the substantial reduction in the amount ofeutectic substance at the grain boundaries is apparent.

When it is desired to extrude one of the billets so treated, it isheated to suitable extrusion temperature and extruded immediatelythereafter in the conventional extrusion press. Generally speaking, Iprefer to maintain the extrusion temperature below about 750 F. but thisdepends upon the character of the alloy being extruded.

Investigation has shown that when using initial pressures of 4000 to4400pounds per square by the conventional method for certainspecificsections. Further comparative tests have shown that the practiceof this invention makes it possible to extrude this type of aluminumbase alloy, using standard extrusion equipment, at considerably lowerextrusion pressures than is possible under conventional extrusionpractice for the same temperature conditions, with less pickup, bettersurface appearance, and higher extrusion speeds.

I claim:

1. A thermal treatment for an ingot of an aluminum base alloy containingsolid solutionforming elements which comprises heating said ingot, priorto working. at a temperature and for a time sufiicient to effectsolution of a material part of said solid solution-forming elements inthe alloy, cooling the ingot at a relatively slow rate to causeprecipitation of a substantial portion of said solid solution-formingelements from solution in the form of relatively large particlesuniformly distributed throughout the alloy, said cooling being continueduntil the ingot reaches a temperature at which substantially no furtherstructural change occurs in the alloy, and then reheating said ingot toa suitable extrusion tem-- perature.

2. A process for extruding an ingot of an forming elements and having acored dendritic structure, which comprises heating said ingot at atemperature and for a time suflicient to effect solution of a materialpart of said solid solutionforming elements in the alloy, cooling theingot at a relatively slow rate to cause precipitation of a substantialportion of said solid solution forming elements from solution in theform of relatively large particles uniformly distributed throughout thealloy, heating said ingot at a relatively rapid rate to a suitableextrusion temperature, and immediately thereafter extruding said ingotto the desired shape.

3. A process for extruding an ingot of an aluminum base alloy containingsolid solution-forming elements and having a cored dendritic structure,which comprises heating said ingot at a temperature and for a timesuiiicient to efiect solution of a material part of said solidsolutionforming elements in the alloy, cooling the ingot at a relativelyslow rate to cause precipitation of a substantial portion of said solidsolutionforming elements from solution in the form of relatively largeparticles uniformly distributed throughout the alloy, said cooling beingcontinued until the ingot reaches a temperature at which substantiallyno further structural change occurs in the alloy, heating said ingot ata relatively rapid'rate to a suitable extrusion temperature, andimmediately thereafter extruding said ingot to the desired shape. I

4. A process for extruding an ingot of an aluminum base alloy containingsolid solution-forming elements including copper and having a coreddendritic structure, which comprises heating said ingot at a temperatureand for a time suflicient to effect solution of a material part of saidsolid solution-forming elements in the alloy, cooling the ingot at arelatively slow rate to cause precipitation of a substantial portion ofsaid solid solution-forming elements from solution in the form ofrelatively large particles uniformly distributed throughout the alloy,heating said ingot at a relatively rapid rate to a suitable extrusionytemperature, and immediately thereafter extrudyfing said ingot to thedesired shape.

a time sufflcient to effect solution of a material part of said solidsolution-forming elements in the alloy, cooling the ingot at arelatively slow rate to cause precipitation of a substantial portion ofsaid solid solution-forming elements from solution in. the form ofrelatively large particles uniformly distributed throughout the alloy,heating said ingot at a relatively rapid rate to a I about 0.1 per centto about 2.0 'per cent magnesium and having a cored dendritic structure,which comprises heating said ingot at a temperature of from about 890 F.to about 950 F. for a time sumcient to effect solution of a materialpart of said solid solution-forming elements in the alloy, cooling theingot at a relatively slow rate to cause precipitation of a substantialportion of said solid solution-forming elements from solution in theform of relatively large particles uniformly distributed throughout thealloy, heating said ingot at a relatively rapid rate to a suitableextrusion temperature, andimmediately thereafter extruding said ingot tothe desired shape.

7. An ingot of an aluminum base alloy containing. solid solution-formingelements and being capable of being extruded at higher speeds or atlower pressuresthan heretofore employed with similar alloys,-said ingotbeing characterized by an internal structure at the hot workingtemperature resulting from a thermal treatment comprising heating saidingot after casting and before working thereof at a temperature and fora time sufficient to eflect solution of a material part-of saidsolution-forming elements in the alloy, cooling the ingot at arelatively slow rate to cause precipitation of a. substantial portion ofsaid solid solution-forming elements from solution in the form ofrelatively large particles uniformly distributed throughout the alloy,and then rapidly heating the ingot to a suitable working temperature.

8. A process for working an ingot of an aluminum base alloy containingsolid solution-forming elements, which comprises heating said ingot at atemperature and for a time suflicient to effect solution of a materialpart of said solid solutionforming elements in the alloy, cooling theingot at a relatively slow rate to cause precipitation of a substantialportion of said solid solutionforming elements from solution in the formof relatively large particles uniformly distributed throughout thealloy, and then working said in-' got containing said large precipitatedparticles to the desired shape.

9. A process for working an ingot of an aluminum base alloy containingsolid solution-forming containing said large precipitated particles torate to cause precipitation of a substantial portion of said solidsolution-forming elements from solution in the form of relatively largeparticles uniformly distributed throughout the alloy, said cooling beingcontinued until the ingot reaches a temperature at which substantiallyno further structural change occurs in the alloy, and then rapidlyreheating said ingot to a suitable working temperature withoutsubstantial solution of said large precipitated particles.

tion-forming elements, copper and magnesium,

in amounts offrom about 2.5 to 6 per cent copper.

- and from about 0.1 to 2 per cent magnesium, and

precipitation of a substantial portion of said solid solution-formingelements from solution in the form of relatively large particlesuniformly distributed throughout the alloy, rapidly reheating said ingotto a suitable hot working temperature and thereafter working said ingotto the desired shape.

THOMAS L. FRI'IZLEN.

